Why are my chillies staying green and not ripening?

Chillies are tropical perennials being grown as annuals in a country where summer lasts about ten weeks. A fruit typically takes several weeks to go from full size to full colour, and every degree of warmth speeds it up. Most of what you can do is about giving the plant more heat and less to do.

Step by step

1. Check the plant is actually warm enough.Chillies ripen well above about 20°C and stall as it cools. A greenhouse, conservatory, porch or sunny windowsill will do far more than the best spot outdoors in most of the UK.
2. Bring outdoor plants under cover in late summer.Moving a pot into a greenhouse or indoors in early autumn often finishes a crop that would otherwise stay green until the frosts.
3. Remove late flowers and tiny fruit.From late summer, pinch off new flowers and any fruit too small to finish. The plant then puts its energy into ripening what it already carries.
4. Reduce the total fruit load if it is very heavy.A plant carrying forty fruit ripens them all slowly. Removing the smallest third speeds up the rest noticeably.
5. Keep feeding with high potash.Potassium supports fruit ripening. Keep going with a tomato feed while fruit is on the plant; do not switch to a nitrogen feed, which pushes new leafy growth instead.
6. Ease off the watering slightly as fruit matures.Slightly drier conditions once fruit is full size encourage ripening and concentrate flavour and heat. Do not let the plant wilt.
7. Give maximum light.Move the pot to the brightest position available and turn it every few days so the whole plant gets the light. Clean greenhouse glass in autumn makes a measurable difference.
8. Ripen the last of the crop indoors.Picked full-sized green chillies will colour on a warm sunny windowsill over a week or two, especially in a paper bag with a ripe banana. They are less intense than plant-ripened fruit but perfectly good.
9. Use the green ones.Green chillies are edible and useful — just fresher and less sweet than ripe ones. Pickle them, freeze them whole, or use them in cooking as they are.

Tips

• Variety matters enormously. Habaneros and other slow, hot types often will not ripen outdoors in the UK at all, while small early varieties ripen readily on a windowsill.
• Sow chillies early — late winter — because the season length is the binding constraint. A March sowing is often too late for the hotter varieties.
• Chillies get hotter as they ripen, so a green fruit from a hot variety is milder than the red one from the same plant.
• Freezing whole chillies works well and keeps the heat; they go soft on thawing but are fine for cooking.
• A plant kept over winter starts the next season months ahead and ripens a much bigger crop.

Common mistakes

• Leaving plants outdoors into autumn hoping for ripening — Cool nights stop ripening altogether. Moving the pot under cover is usually worth several weeks.
• Letting the plant keep flowering into September — It spends energy on fruit that will never finish. Removing late flowers concentrates it on the crop you can actually have.
• Feeding nitrogen to 'perk it up' in late summer — It produces leaves and delays ripening. High potash is the right feed at that stage.
• Picking too early and expecting full heat — Both colour and capsaicin develop as the fruit matures. Green fruit from a hot variety is noticeably milder.

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